A sinking fund is a small amount you save every month toward a specific expense you already know is coming, an annual insurance premium, a car repair, holiday spending, so the bill never shows up as a surprise. A $600 yearly premium becomes $50 set aside monthly instead of a shock in the month it's due.
Want the full method? See what are sinking funds and how do they work for how to set one up, where to keep the money, and how it's different from an emergency fund.